1. Field of Invention
This invention pertains to the field of optical fibers.
2. Description of Prior Art
There are several conventional processes for making glass optical fibers, including, rod-in-tube, nozzle tube and double crucible processes.
In the rod-in-tube process, a glass core rod is placed into a glass cladding tube, heated to above the glass softening temperature, and drawn into an optical fiber. Typically, the core rods are obtained by quenching the core melt whereas the cladding tubes are obtained by spinning the cladding melt during cooling. However, several problems are associated with the rod-in-tube process that limit its usefulness. The problems include sublimation of the glass components at fiber draw temperatures and subsequent deposition on the rod and inner surface of the tube and presence of capillaries and imperfections at the coreclad interface. As a result, the fibers drawn by this process possess high optical loss due to scattering of light and are weak. Also, the fiber core/clad ratio is fixed and predetermined by the rod and tube diameters.
The nozzle tube process is a modification of the rod-in-tube process. In this process, the glass rod and tube are placed into a nozzle tube and drawn into a fiber. The core/clad interface is under vacuum and the outside of the cladding tube is pressurized so that the fiber can be drawn at a lower temperature. Lower temperature minimizes the sublimation of the glass components but does not completely remove it, since the vacuum enhances sublimation. Therefore, the fiber optical losses remain high and the fibers are relatively weak. Again, the fiber core/clad ratio is typically fixed and predetermined by the rod and tube diameters.
In the double crucible process, the core and cladding glass cullets or powders are charged into two concentrically disposed crucibles with an orifice in their bottom portion. The crucibles are placed into a resistance furnace and the glasses are melted and fined for several hours. Then, the temperature is dropped slowly to the fiberizing temperature and the melt is drawn into an optical fiber. There are several disadvantages in this process which can be attributed to the different processing steps. The disadvantages include contamination of the glass during the fracturing and grinding of the glass chunks, trapping of gas bubbles during melting, volatilization losses which change the composition of the glasses due to a long fining time and no control over the core/clad diameter ratio.